code: def _(y) -> Optional[str]:
      return _infer_freq_from_index(get_time_index(y.as_list()[0]))
  
file_path: sktime/sktime/utils/datetime.py
imports: import warnings
  from functools import singledispatch
  from typing import Optional, Tuple, Union
  import numpy
  import pandas
  from sktime.datatypes import VectorizedDF
  from sktime.datatypes._utilities import get_time_index
  from sktime.utils.validation.series import check_time_index, is_integer_index
label_desc: Use image node_id 7 for calling a global function with example usage: _(y) and returns: _infer_freq_from_index